Fan noise has been identified as a primary component of overall noise generated by various types of machinery, including heat exchanger equipment. For example, low speed, low pressure axial flow fans are typically used in heat exchanger applications, such as for moving ambient air over commercial and residential air conditioning condenser heat exchangers. In residential air conditioning systems, low speed, low pressure axial flow fans typically meet the requirements for effective operation in terms of performance capability, durability, and cost.
Although relatively low speed, low pressure axial flow fans have achieved noticeable reduction in noise generation through the design of the fan blading and reductions in turbulence from motor supports and fan shrouding, many of such fans continue to generate noise at frequencies which are perceived by the human ear as somewhat annoying. Moreover, the application of axial flow, low speed, low pressure fans in residential air conditioning systems, where relatively high density dwellings result in a condenser unit for one residence being within a few feet of an adjacent residence, has mandated further reductions in noise generated by air conditioning condenser cooling fans, in particular.
Fan self induced tonal noise in a frequency range of about 2300-3500 Hz has been identified during operation of low speed, low pressure, axial flow fans. Reduction of noise in this frequency range as well as over a relatively broad range of frequencies normally audible to humans is always sought. One source of noise in axial flow fans, in particular, is due to a phenomenon known as laminar boundary layer shedding. This phenomenon is similar in some respects to the generation of the well-known von Karman vortex streets which occur when fluid flows around a body disposed in the fluid flow path. In accordance with the present invention, tonal noise generated by laminar boundary layer shedding has been measurably decreased thereby providing advantages in fans used in various air-moving applications and, particularly, in applications associated with heat exchange equipment in air conditioning systems and the like.